As one of the measures for fuel economy improvement of automobiles beginning from global environmental problems, automobile body lightening has proceeded, and steel sheets to be used for automobiles need to be strengthened as highly as possible. However, highly strengthening of steel sheets for automobile lightening lowers elongation EL or r value (Lankford value), resulting in the deterioration of press formability or shape fixability.
To solve such a problem, a hot press-forming method has been adopted for production of parts, in which method a steel sheet is heated to a prescribed temperature (e.g., a temperature for change in austenite phase) to lower its strength (i.e., make it easily formable) and then formed with a press tool at a temperature (e.g., room temperature) lower than that of the thin steel sheet, whereby the steel sheet is provided with a shape and at the same time heat treated by rapid cooling (quenching), which makes use of a temperature difference between both, to secure its strength after forming.
According to such a hot pressing method, a steel sheet is formed in a state of low strength, and therefore, the steel sheet has decreased springback (favorable shape fixability). In addition, the use of a material having excellent hardenability, to which alloy elements such as Mn and B have been added, thereby obtaining a strength of 1500 MPa class in terms of tensile strength by rapid cooling. Such a hot press-forming method has been called with various names, in addition to a hot press method, such as a hot forming method, a hot stamping method, a hot stamp method, and a die quench method.
FIG. 1 is a schematic explanatory view showing the structure of a press tool for carrying out hot press forming as described above (hereinafter represented sometimes by “hot stamp”). In this figure, reference numerals 1, 2, 3, and 4 represent a punch, a die, a blank holder, and a steel sheet (blank), respectively, and abbreviations BHF, rp, rd, and CL represent a blank holding force, a punch shoulder radius, a die shoulder radius, and a clearance between the punch and the die, respectively. In these parts, punch 1 and the 2 have passage 1a and passage 2a, respectively, formed in the inside thereof, through which passages a cooling medium (e.g., water) can be allowed to pass, and the press tool is made to have a structure so that these members can be cooled by allowing the cooling medium to pass through these passages.
When a steel sheet is subjected to hot stamp (e.g., hot deep drawing) with such a press tool, the forming is started in a state where steel sheet (blank) 4 is softened by heating to a temperature within two-phase region, which is from Ac1 transformation point to Ac3 transformation point, or a temperature within single-phase region, which is not lower than Ac3 transformation point. More specifically, steel sheet 4 is pushed into a cavity of die 2 (between the parts indicated by reference numerals 2 and 2 in FIG. 1) by punch 1 with steel sheet 4 in high-temperature state being sandwiched between die 2 and blank holder 3, thereby forming steel sheet 4 into a shape corresponding to the outer shape of punch 1 while reducing the outer diameter of steel sheet 4. In addition, heat is removed from steel sheet 4 to the press tool (punch 1 and die 2) by cooling punch and die in parallel with the forming, and the hardening of the material is carried out by further retaining and cooling steel sheet 4 at the lower dead point in the forming (the point of time when the punch head is positioned at the deepest level: the state shown in FIG. 1). Formed products with high dimension accuracy and strength of 1500 MPa class can be obtained by carrying out such a forming method. Furthermore, such a forming method results in that the volume of a pressing machine can be made smaller because a forming load can be reduced as compared with the case where parts of the same strength class are formed by cold pressing.
As steel sheets for hot stamp, which have widely been used at present, there are known steel sheets based on 22MnB5 steel. These steel sheets have tensile strengths of 1500 MPa and elongations of about 6% to 8%, and have been applied to impact-resistant members (members neither deformed nor fractured as much as possible at the time of impact). In addition, some developments have also proceeded for C content increase and further highly strengthening (in 1500 to 1800 MPa class) based on 22MnB5 steel.
However, there is almost no application of steel grades other than 22MnB5 steel. One can find a present situation where little consideration is made on steel grades or methods for controlling the strength and elongation of parts (e.g., strength lowering to 980 MPa class and elongation enhancement to 20%) to extend their application range to other than impact-resistant members.
In middle or higher class automobiles, taking into consideration compatibility (function of, when a small class automobile comes to collide, making safe of the other side) at the time of side or back impact, both functions as an impact-resistant portion and an energy-absorbing portion may sometimes be provided in parts such as B pillars or rear side members. To produce such members, there has mainly been used so far, for example, a method in which ultra-high tensile strength steel sheets having high strength of 980 MPa class and high tensile strength steel sheets having elongation of 440 MPa class are laser welded (to prepare a tailor welded blank, abbreviated as TWB) and then cold press formed. However, in recent years, the development of a technique has proceeded, in which parts are each provided with different strengths by hot stamp.
For example, Non-patent Document 1 has proposed a method of laser welding 22MnB5 steel for hot stamp and a material that does not have high strength even if quenched with a press tool (to prepare a tailor welded blank, abbreviated as TWB), followed by hot stamp, in which method different strengths are provided so that tensile strength at a high strength side (i.e., impact-resistant portion side) becomes 1500 MPa (and elongation becomes 6% to 8%) and tensile strength at a low strength side (i.e., energy-absorbing portion side) becomes 440 MPa (and elongation becomes 12%). In addition, as the technique of providing parts each with different strengths, some techniques have also been proposed, such as disclosed in Non-patent Documents 2 to 4.
The techniques disclosed in Non-patent Documents 1 and 2 provide a tensile strength of not higher than 600 MPa and an elongation of about 12% to 18% at an energy-absorbing portion side, in which techniques, however, laser welding (to prepare a tailor welded blank, abbreviated as TWB) is needed previously, thereby increasing the number of steps and resulting in high cost. In addition, it results in the heating of energy-absorbing portions, which need not to be hardened originally. Therefore, these techniques are not preferred from the viewpoint of energy consumption.
The technique disclosed in Non-patent Document 3 is based on 22MnB5 steel, in which boron addition, however, adversely affects the robustness of strength after quenching against heating to a temperature within two-phase region, making difficult the control of strength at an energy-absorbing portion side, and further making it possible to obtain only an elongation as low as 15%.
The technique disclosed in Non-patent Document 4 is based on 22MnB5 steel, and therefore, this technique is not economic in that control is made in such a manner that 22MnB5, which originally has excellent hardenability, is not hardened (control of press tool cooling).